When the Tutorial prompts you to spend a bone in order to speed up the completion of the building. Disregard the instruction because it will still be completed fast and wait for a couple of seconds you will save a bone.

Although the game tries to get you to spend bones on rushing building, the longer you leave it the less bones you have to spend. In the tutorial, for example, when prompted to rush construction of the dog room - if you ignore it, it will be built quickly and save you a bone.